1. Field of the Invention
This invention relates to the field of operational amplifier test circuits, and particularly to test circuits which determine an op amp's input offset voltage (VOS) and input bias current (IB).
2. Description of the Related Art
The parameters of an operational amplifier (op amp) are typically established by means of a test circuit which applies known inputs to the op amp's inverting and non-inverting inputs and monitors its output. For example, input offset voltage (VOS), i.e., the voltage that results in an output of zero when applied to both op amp inputs, is typically determined by connecting the inverting and non-inverting inputs of the device under test (DUT) to ground via respective resistors, and connecting a feedback resistor between the DUT's output and its inverting input. The input and feedback resistors are generally selected to establish a high gain around the DUT. VOS for the DUT is determined by measuring its output voltage under these conditions, and dividing the measured voltage by the gain value.
Another frequently tested op amp parameter is input bias current (IB), i.e., the current which flows into each of the DUT's inputs when it is operating. This can be determined in any of several ways. For example, a current meter may be connected in series with each of the DUT's inputs and IB measured directly. Alternatively, IB can be determined by measuring the voltage across a pair of input resistors with a very sensitive voltmeter.
Many op amps must be tested for both VOS and IB. When this is the case, IB can be determined by measuring VOS as described above, and then inserting an additional resistance in series with one of the DUT's inputs and measuring VOS again. The difference between the two VOS readings is divided by the additional resistance to give IB.
Virtually all of these test strategies require the use of some sort of switching circuitry, to manipulate the test conditions and to make appropriate measurements as necessary. For example, when IB is measured by comparing VOS measurements with and without the presence of an additional input resistance, a switching means must be used to insert the resistance into and remove it from the circuit. Semiconductor switches can be used for this purpose, but doing so tends to degrade the accuracy of the testing, due to the inherent resistance of such a switch, together with the time required to fully turn on or off, and their leakage current in the off condition. While this may not be a problem for general purpose op amps, it is unacceptable when testing precision op amps, which can have VOS and IB specifications of less than 10 .mu.V and 1 nA, respectively.
Conventional electromagnetic relays are often employed as a switching means for testing precision op amps, due to the very low resistance conductive path provided when their contacts are closed. However, when the electromagnetic coil of a conventional relay is energized for a length of time, heat from the coil generates a thermal E.M.F. in series with the relay contacts. This thermal voltage represents an error that gets added directly to the DUT's VOS measurement, degrading the accuracy of both VOS and IB measurements. When very low VOS and/or IB values must be measured, the error due to thermal E.M.F. can be unacceptably high.